Paper is manufactured in widths of up to four-hundred inches. The paper web must be uniform across its entire width. The width of the paper web, combined with the requirement for uniformity, means that the components of a papermaking machine must maintain dimensional stability across the width of a paper web.
Components of a papermaking machine where dimensional stability is particularly important include the head box, which must uniformly distribute paper fibers under the fourdrinier paper-forming screen, quarter heads, size press heads, the support beam for induction heaters, and the supporting framework for cross machine scanners, as well as cooled reel drums.
One area where dimensional stability is particularly important is in the support of doctor blades which are used in applying coatings to the web as well as for cleaning roll surfaces and, in the case of the Yankee dryer, removing the dried web from the dryer surface. To function properly, doctor blades must engage the surface of the roll or the surface of the web uniformly over the entire length of the web. A problem associated with thermal distortion of the doctor blade support makes retaining dimensional stability across the width of the papermaking machine difficult. Doctor blades are invariably utilized on heated rolls forming part of a dryer. Thus, the blade and its support structure are inevitably heated by radiative heat transfer from the roll or the overlying web to the exposed surfaces of the doctor support structure. Radiative heat transfer has an effect on only the exposed or illuminated portions of the support structure, thus inducing uneven heating within the structure. This leads to thermal distortion and lack of dimensional stability.
One solution to thermal distortion in the doctor blade support structure is the simple and expedient method of applying steam heating to the structure so that all the surfaces comprising the support structure are held at a uniform temperature by the condensation of steam thereon. However, this process is wasteful of energy and requires constantly supplying and controlling the steam pressure and quality supplied to the doctor blade support structure.
What is needed is a self-contained doctor support structure which prevents thermal gradients from forming within the support structure and is thus structurally stable when used in proximity to heated surfaces.